This disclosure relates to circuitry for use in transmitting a digital (e.g., binary) signal from one circuit or system component to another. A typical (but not necessarily the only) application of this disclosure is in circuitry for transmitting a high-speed serial data signal from one integrated circuit (“IC”) on a printed circuit board (“PCB”) to another IC on that PCB or to another IC on another PCB that is connected to the first PCB via a so-called back-plane (“BP”) circuit structure. Because some aspects of the disclosure may relate to circuitry that is configurable (e.g., programmable) for use in a range of different applications, the circuitry of this disclosure may be provided on a general-purpose or relatively general-purpose IC such as a programmable logic device (“PLD”), a field-programmable gate array (“FPGA”), a programmable microcontroller, or the like.
A high-speed serial data signal (e.g., a serial data signal typically (but not necessarily) having a binary digit (“bit”) rate in excess of 1 gigabits per second (“1 Gbps”)) that needs to be transmitted by one circuit component (e.g., a first IC) to another circuit component (e.g., a second IC) is typically subject to some attenuation (loss, distortion) as it travels through the transmission medium from the transmitting (transmitter, “TX”) component to the receiving (receiver, “RX”) component. Such attenuation tends to be exponentially greater for the higher frequency components of the signal being transmitted. Attenuation makes it more difficult for the receiver component to accurately interpret (recover) the data from the signal it receives.
To reduce the adverse effects of the above-described signal transmission attenuation, it is known to give the high-speed serial data signal what is sometimes called pre-emphasis just prior to transmitting the signal. For example, such pre-emphasis may give each data bit in the signal that has binary data value (e.g., binary 1 or binary 0) different from the immediately preceding bit some extra energy (e.g., greater voltage) in the high-speed serial data signal as that signal emanates from the transmitter. The high-speed serial data signal needs its highest frequency components to convey such changes in binary value; and because these frequency components are subject to the greatest transmission loss (attenuation), giving these binary value changes extra energy helps to compensate (in advance) for the transmission loss they are especially vulnerable to. The signal therefore arrives at the receiver in better condition, and the receiver is consequently better able to accurately recover the data contained in that signal.
Typically, if transmitter circuitry is able to operate at any one of several bit rates over a range of possible bit rates (e.g., a bit rate (in such a range) that is selectable (configurable, programmable) by a user of the circuitry), only one type of pre-emphasis may be available for use regardless of the bit rate that is selected for use. That type of pre-emphasis may work well for bit rates up to a particular bit rate threshold. But it may not work as well for bit rates greater than that threshold.